1. Technical Field
The present invention relates to a thermoelectric material, a method for preparing the same, and a thermoelectric module including the same.
2. Description of the Related Art
A rapid increase in use of fossil energy may cause global warming and energy depletion. Recently, programs relating to reproducible energy development and thermoelectric device development have actively progressed all over the world, including Korea, in order to address these problems. Particularly, since all equipment and electronic instruments do not overcome the limitations of Carnot cycle thermodynamically, waste heat makes up a majority of input energy. Therefore, it is preferable to reuse heat energy and apply it to a new field in order to overcome the energy crisis.
Thermoelectric devices and modules are largely divided into an electricity generation field using Seebeck effect and a cooling field using Peltier effect. In the cooling field, dissipation of heat is becoming increased due to smallness, high power consumption, high integration, and slimness of electronic parts in addition to development of IT industries, and the heat serves as an important factor that causes malfunction and decrease in efficiency of the electronic instruments.
A thermoelectric device is used in order to solve these problems, and future applicability of the thermoelectric device will be further enlarged, considering advantages of the thermoelectric device such as non-noise, high cooling rate, local cooling, and eco-friendly features.
Also in thermoelectric generation field, many efforts to regenerate electric energy by using a large quantity of waste heat discharged from vehicles, waste incinerators, steelworks, power plants, geothermal heat, electronic instruments, body heat, or the like has been made throughout the world. Particularly, thermoelectricity generation is bulk electricity generation, and can be fused into other electricity generation, and thus future applicability thereof is very high. Further, thermoelectricity generation does not cause earth polluting materials while generating electric energy, and thus it is consistent with eco-friendly features. Therefore, the propagation rate of thermoelectricity generation will be accelerated.
However, commercialization of thermoelectric cooling and thermoelectricity generation are not common throughout the world, and studies have been conducted only at a national laboratory or an academic laboratory. However, the thermoelectric devices and modules have been further studied in order to solve a recent increase in energy cost and environmental problems. Therefore, a future market thereof will be enlarged considering applicability thereof.
FIG. 1 shows a structure of a module part of a thermoelectric device excluding a power source part, which is currently used. A thermoelectric device largely employs a N-type semiconductor 11 and a P-type semiconductor 12, and further consists of a metal electrode 13 connecting the N-type semiconductor 11 and the P-type semiconductor 12, and a ceramic substrate 14, which is called a single module.
In order to allow the single module to be used as a cooling device or an electricity generator device, charges are generated in the N-type semiconductor 11 and the P-type semiconductor 12, and then respective terminals therefore need to be connected to a circuit by metal electrodes 13.
Therefore, in order to increase efficiency of the single module, the module needs to be designed such that respective components constituting the module have high efficiency, and mutual efficiency between the respective components is at the optimum. However, due to low conversion efficiency of the single module, there is a need of employing a composite module using several single modules, in an applicable field of thermoelectric module.
The conventional composite module is manufactured by connecting single modules of p-n constitution in series according to the working condition. Respective single modules are connected by the metal electrode, and the metal electrode is connected to the ceramic substrate. Since the respective single modules are designed to be parallel with each other from a heat source, the temperature gradient of semiconductor material per se is equal between the single modules.
This existing series connection type module structure has problems with respect to disconnection of circuit, and has a fatal risk that the composite module can not be operated overall when any one of the single modules constituting the composite module breaks down. Further, this series connection type module has a large disadvantage in that voltage dependence is large.
Meanwhile, the thermoelectric material of the related art was synthesized by ingot growing or mechanical alloying. The thermoelectric material prepared as above, as shown in FIG. 2, has a structure in which particles each have a size of several μm and a spherical shape and they are randomly distributed without a certain rule.
Recent studies on the thermoelectric material are proceeding in a direction of reducing heat conductivity due to phonon scattering. For this phonon scattering, the size of the thermoelectric material needs to be smaller than wavelength of the phonon, but generally, it has a size of several tens of μm.
Therefore, many developers attempt to synthesize a nanometer-sized material capable of inducing phonon scattering to reduce heat conductivity, but the results have not been sufficient until now.